The Background described in this section is included merely to present a general context of the disclosure. The Background description is not prior art to the claims in this application, and is not admitted to be prior art by inclusion in this section.
The Global Positioning System (GPS) is a global navigation satellite system (GNSS) that includes devices implemented to receive satellite data signals. For example, hand-held navigation devices, vehicle navigation devices, as well as navigation-enabled devices, such as mobile phones and computer devices, are increasingly more common. The satellite data signals are transmitted from satellites of the GNSS, and the satellite data signals include an accurate satellite time reference generated by an atomic clock on each satellite, ephemeris data that includes orbital information for a satellite, and the almanac information that includes approximate orbital and status information for all of the satellites in the system.
A time to first fix (TTFF) is a measure of the time duration, or time lapse, for a GPS receiver to acquire satellite data signals and calculate a position solution, referred to as a position fix of the receiver. Generally, a fast time to a first position fix is a desired parameter or characteristic of GPS-enabled devices when the position of a device is determined and calculated from the satellite data signals. A GPS receiver can take time to acquire each GPS satellite data signal, and the time lapse may range from thirty seconds to several minutes for satellite signal acquisition and tracking to then calculate a solution for the receiver position fix.
To calculate a solution for a position fix, a GPS receiver uses the satellite data signals to determine a precise time of signal transmission from each satellite, and then computes a distance to each satellite. The time difference between satellite signal reception and transmission provides the receiver with information to determine the range to a transmitting satellite. Based on the computed distances and the position of each satellite in orbit, a GPS receiver can determine the geographic position of the receiver and display the determined position for a user, such as on a hand-held navigation device or in a vehicle. Typically, the satellite data signals from three satellites are used to determine position along with the satellite data signal from a fourth satellite that is used to determine a correction for time error.
A common and significant source of the time error is the clock in a GPS receiver. Because the speed of light is such a large value when determining the precise time of signal transmission from each satellite, even a one microsecond clock error of the GPS receiver clock can result in a position determination error of almost a thousand feet. Ideally, a GPS receiver implemented with an atomic clock would keep precise time for position determination. However, the expense of an atomic clock would likely double the cost of GPS-enabled devices.
Rather than using atomic clocks in GPS-enabled receiving devices, the distance to each satellite is based on a pseudo-range, which is an approximation of the distance between a satellite and the GPS receiver, because the calculated distances to the three satellites are all based on the same approximate clock error. The distances, or ranges, with the same approximate clock error are the pseudo-ranges. The time error can then be estimated from the determined pseudo-ranges and an additional fourth satellite. The geographic position of a GPS receiver can be accurately computed from the position of the receiver along the x, y, and z axes (e.g., latitude, longitude, and altitude) with a time error Δt based on the pseudo-ranges and orbit positions of the four or more satellites.